1. Field of the Invention
The present invention relates to a microwave integrated circuit.
2. Description of the Prior Art
In recent years, development of various devices using a microwave circuit such as a local wireless LAN, a millimeter wave radar and the like has been made. In this development, the most significant matter is to simply and cheaply manufacture the microwave circuit. In order to reduce the manufacture cost of the microwave circuit, it is the most effective means to make small the dimension of a chip with the microwave circuit. Thus, the circuits which are not generally employed are of the type that is called a monolithic microwave integrated circuit (MMIC). As shown in FIG. 1, the monolithic microwave integrated circuit is a circuit having a micro strip line structure in which electronic devices such as MESPET, HEMT and the like and a transmission line such as a micro strip line and the like are integrated together on a GaAs substrate. In the MMIC chip, it is possible to make small the dimension of the chip. However, the manufacture process of the MMIC chip is difficult and its yield is also low. More specifically, as shown in FIG. 1, the micro strip line structure, in general, has a signal line 5 which is formed on one side or surface side of the GaAs substrate 1 and a ground conductor 3 which is formed on the opposite side or the back side thereof. However, the process for forming this structure is very difficult, because the GaAs substrate usually having 650 micron must be made thin up to a thickness of a degree of 50 micron so as to form a micro strip line between the surface and back sides of the substrate. As a result, the strength of substrate is reduced. Moreover, in order to make contact with the ground conductor 3 on the back side of the substrate, a via hole must be formed in the GaAs substrate having a thickness of 50 micron. However, this formation process is also very difficult.
Taking such circumstances into consideration, there has been proposed a method of forming a thin film micro strip line on the GaAs substrate (this structure is called reverse micro strip line structure, ref. FIG. 2), in order to readily form the micro strip line. In this method, a dielectric film 7 made of resin is formed on the surface of the GaAs substrate 1, and the ground conductor 3 is further provided on the dielectric film 7, thereby constructing a micro strip line using the dielectric film 7 as an interlayer dielectric. With the construction as described above, there is no need of making thin the GaAs substrate 1 and forming a via hole in the GaAs substrate 1. Therefore, the manufacture process is easy. However, the dielectric film is formed on the electronic device such as HEMT and MESFET. In accordance with this structure, in order to connect the micro strip line with these electronic devices, there is a need of forming a contact hole in the interlayer dielectric film 7, as shown in FIG. 2 (hereinafter, reference numerals G, D and S in the accompanied drawings denote a gate electrode, a drain electrode and a source electrode of electronic device, respectively). Then, if the thickness of the dielectric film 7 becomes thick, the contact hole must be formed large. As a result, the dimension of the MMIC type chip also becomes large. Moreover, in the reverse micro strip line structure, the signal line 5 and the dielectric film 7 with specific dielectric constant of being about 3 are formed on the GaAs substrate with dielectric constant of being 13, and further, the ground conductor 3 is formed thereon as the uppermost layer. In the structure as described above, an electric field is generated as shown by an arrow E of FIG. 3. In this state, effective dielectric constant changes depending upon interconnecting pattern. Then, if the effective dielectric constant changes, an effective wavelength also changes. Due to the changes as described above, a design of microwave circuit cannot help being complicated.
Meanwhile, in order to solve the above problem, there has also been proposed an MMIC chip which employs a thin film micro strip line having a normal micro strip line structure as shown in FIG. 4. In this structure, the ground conductor 3 is formed adjacent to the GaAs substrate 1. Therefore, an electric field E is substantially closed between the signal line 5 and the ground conductor 3, as shown in FIG. 5. Accordingly, effective dielectric constant is determined substantially by the interlayer dielectric film 7. Therefore, a design of microwave circuit is easy. However, this structure also needs to form a contact hole in the dielectric film 7 in order to mutually connect the signal line 5 of the micro strip line and the electronic device, and, if the dielectric film 7 is formed thick, the contact hole must be also formed larger. In accordance with this, as shown in FIG. 6, a distance d from the electronic device to the micro strip line end becomes long in order to secure a margin between contact holes. The distance d is hard to handle as a model when making a design of circuit, and it produces inductive reactance which cannot be neglected when a millimeter wave band having a short wavelength, etc. is utilized. Therefore, it is difficult to make an alignment of electronic devices with interconnecting liens. Moreover, in the case where the dielectric film is formed on electronic devices such as HEMT and MESFET, gate-to-source capacitance and gate-to-drain capacitance increase. As a result, high speed performance of these electronic devices deteriorates.
As is seen from the above description, in the MMIC chips having electronic devices such as HEMT and MESFET and a surface micro strip line using resin, to achieve cost reduction, it is desirable that the distance from electronic device to the micro strip line end is made short as much as possible so as to make small chip size and to enhance alignment with a design.